Xolography—The Latest Innovation in 3D Printing

3D printing is entering its fourth decade. The technology has been booming lately—hundreds of millions of dollars are being invested in industrial 3D printing, with companies exploring applications ranging from footwear and implants to rocket engines.

There are several types of 3D printing. The most popular is currently fused deposition modeling (FDM)—also known as fused filament fabrication (FFF)—a cost-effective and quick method for producing physical models. 

Enter volumetric 3D printing, also known as holographic or tomographic 3D printing, which was first introduced by researchers at Lawrence Livermore National Laboratory (LLNL). Instead of a layer-by-layer approach, volumetric 3D printing works by repeatedly projecting a pattern into a vat of transparent photopolymer liquid from various angles; this method is known as computed axial lithography (CAL). It is like a CT scan in reverse, and the pattern is projected to form the object rather than scanning the object.

The researchers introduce xolography as “a dual-color technique using photoswitchable photoinitiators to induce local polymerization inside a confined monomer volume upon linear excitation by intersecting light beams of different wavelengths.” The concept is demonstrated using a volumetric printer designed to generate 3D objects with complex structural features as well as mechanical and optical functions.

In other words, two different kinds of light energy are applied on a vat of photopolymer resin that reacts differently to the light energy. This also explains the name of the company and the printing process, as the crossing (X) light beams generate entire objects (holos).

The approach is to project a “sheet” of light into the resin vat. Though sheets may seem much like the layers used in other 3D printing, there are many different angles in this approach. The sheets are not stacked on top of each other to build an object. As can be seen in the figure above, a light sheet of a certain wavelength (λ₁) activates a thin layer of photoinitiator molecules. Another projector arranged orthogonally generates light of a different wavelength (λ₂) and focuses sectional images of the 3D model to be manufactured into the plane of the light sheet. Only the initiator molecules in the latent state absorb the light of the projector and cause the activated resin to solidify as it polymerizes. The optical setup is fixed, while the vat of resin is moved to present different views. Additionally, because the rest of the resin is not activated, the intersecting wavelength inhibits polymerization. In this way, the desired object is continuously fabricated.

Because this approach of utilizing two intersecting lights both encourages and inhibits polymerization, xolography can produce solids of very high resolution. Additionally, the sheet approach ensures that any specific voxel is exposed to the curing light only once during the entire process. 

The DCPI is critical for xolography and is made by integrating a benzophenone type II photoinitiator (molecules that react to radiation) and a spiropyran photoswitch (organic chemical compounds with photochromic properties). The resulting polymer efficiently combines the photoswitching and photoinitiating properties with favorable spectral and thermal characteristics. The initial spiropyran state absorbs the first wavelength of 375 nm, which is completely transparent. The polymerization is then initiated when the absorption of the second visible wavelength (450 to 700 nm) excites the benzophenone in combination with the co-initiator. If it is not hit by the visible light, it reverts back to the initial resin state. The result is a very rapid polymerization of the 3D model to be manufactured.

Another advantage of the methodology is that it removes the need for support structures, unlike with other 3D printing techniques. This is because the surrounding viscous resin temporarily supports any loose parts. As stated in the Nature paper: “The crosslinking of monomers leads to changes in density, which results in different sink rates of parts under gravity. The high printing speed and viscosity of the resin minimize this effect, so sinking only becomes apparent after fabrication has been completed.”

With the current setup, xolography can print at a rate of about 55 cubic millimeters per second with structures as small as 25 microns wide. Another volumetric technique, two-photon photopolymerization, is able to print objects with a resolution of less than 100 nanometers, although the process is far slower. CAL is much faster but can only achieve a resolution of 300 microns. Because of the techniques employed, xolography is 10,000 to 100,000 times faster. 

Using two different cross-sectional wavelengths of light resolves the problem of ill-defined solidification that plagues other volumetric printing techniques, because only activated resin is polymerized. The resulting resolutions are approximately 10 times higher when compared to CAL-printed objects while being just as fast, if not faster. 

The xube

Xolo has also designed what appears to be the first purchasable volumetric 3D printer, which it is calling the “xube.” The device has not been released yet but can be reserved on the company’s website. The printer is currently only available for use by researchers in academia and is not geared toward commercial applications.

The xube is a small machine of only 50 x 50 x 50 cm with a build of 50 x 70 x 90 mm, so it’s certainly not yet adequate for additive manufacturing. It includes dual 405 nm lasers and UHD DLP for the projection of each image, and has an optical resolution of 0.03 mm on the x- y-axes, and 0.05 mm on the z-axis. A Python program running on a Raspberry Pi 4 system controls the laser, linear axis and projector.

The outstanding feature of the xube is the listed typical print time: 20 seconds to 5 minutes! Traditional 3D printing usually takes a minimum of 30 minutes, so xolography is definitely many orders of magnitude faster.

The scientists believe that printing speed can be substantially increased if lasers of higher wattage and photoinitiators of faster thermal relaxation time are utilized.

Furthermore, the size of the print volume is only limited in the x-axis, so theoretically large objects can be produced either in a stationary volume or in combination with continuous resin flow systems of unlimited size.

Xolography can also potentially be used for generating complex biological structures using fluids laden with living cells, according to study coauthor Stefan Hecht, a chemist and materials scientist at Aachen University in Germany. Hecht notes that existing bioprinting techniques cause cells to experience stress when they are sprayed out of bioprinter nozzles, which could damage them. This is something that would be avoided when using xolography.

“On the other hand, we can also print extremely hard stuff—we can print glass,” Hecht added. “We can work with an amazing versatility of materials.”

In addition, more complex resins could be used to print multiple kinds of materials at the same time to produce devices such as sensors and electronics. Future research will explore how to remove any resin still remaining within printed items. Hecht and his team are also looking into whether the liquid could be reused.

Xolo’s tagline is “Instantly, it’s there,” and the company seems to be able to deliver. Undoubtedly, the technology is still in the nascent stages. It will face teething problems as all new technologies do. Nevertheless, xolography has definitely caused a buzz in the market and seems to be here to stay.